Krita: http://www.koffice.org/krita/
I have heard a bit about this, while I don't actually use it. It is supposed to have a more sane environment than the Gimp, supports color profiles, and is soon to have natural media brushing, by some other name (because that's trademarked).

GSumi: http://www.gtk.org/~otaylor/gsumi/
I use this for all my sketching, doodline, meandering, hand exercises, and even writing sometimes. It is a BW only drawing tool, with unbelievably tiny memory usage and pressure curve support for more natural usage. Its name comes from the japanese word for ink... according to its website. Note: it requires the old GTK, which must be manually compiled for tablet device support. I reccomend installing packaged versions of all the GTK requirements (ex: pango, atk) to reduce the number of complications. GTK 1 should be able to be installed parallel to GTK 2 with no problems.

These are both for Linux, but with enough effort they could work on Windows too.

Artweaver is a Windows freeware program to simulate natural brush tools. It is therefore suitable to leave your creativity free run. Artweaver offers you a clear program window, which can be used without training immediately.

Sorry to bump this thread, but I noticed you don't have Sketchup on your list of 3D programs. Great for architecture, but no NURBS (ie, curves). And yes, OpenCanvas is free (as are all opensource programs).

kaerhon: Just one time I could run open canvas 4 on linux, it only run on an old version of wine, earlier versions have been impossible to make it run on wine. (in ubuntu at least, haven't tried it on gentoo, but I assume the same result)

But according to wine HQ is possible to have it running without hassle.

For linux I recommend using krita for line art (just make some custom brushes on gimp first) and GIMP for coloring.

inkscape is a servicable opensource replacement for illustrator. can do some stuff that illustrator cannot.

Also good is scribus, which is opensource and opreates much like inDesign for publishing and design.

Both are freeware.

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It was but yesterday I thought myself a fragment quivering without rhythm in the sphere of life. Now I know that I am the sphere, and all life in rhythmic fragments moves within me. -Sand and Foam Khalil Gibran

I know Pixia ( http://park18.wakwak.com/~pixia/ ) has been mentioned before. Just wanted to point out that version 4 came out in English a few months ago.
Pixia is a nice enough freeware program for drawing and painting and it has one feature that I absolutely love: excellent line smoothing. This is something that's right up at the top (together with more advanced brush settings) of my wishlist of things to be implemented in Photoshop. With a bit of practice you can get very nice smooth lines even with a mouse using Pixia. I haven't done much in the way of line art lately, but awhile back I regularly sketched in Photoshop and then went over to Pixia for inking.